Many communications systems allocate separate frequency bands for signals from different sources. One such example is a bidirectional cable (“CATV”) system with a forward path from a head end to one or more subscribers and a return path from the subscribers to the head end. Signals on the forward path may include broadcast television channels. Signals on the return path may include data from a subscriber's cable modem. It is common for signals on the forward path of a CATV system to be transmitted in one frequency band and for signals on the return path to be transmitted in a second frequency band which is lower than and does not overlap with the first frequency band.
CATV systems are commonly designed to provide proper signal levels on the forward path at each subscriber tap. In a bidirectional CATV system, insertion loss in the return path may differ widely among taps. Performance of bidirectional systems can be significantly improved by appropriate attenuation of the return path signals. The attenuation reduces ingress noise and improves the signal to noise ratio of the cable modem. In particular, performance may improve if selective attenuation levels can be applied at various points of the return path.
CATV systems may have more than one source of return path signals, each source transmitting in a different frequency range. In one common example, a subscriber has both a cable modem and a set top box receiver for digital cable television service. The set top box may transmit information, for example, indicating that a subscriber wishes to purchase a video-on-demand program. The frequency band for signals from the set top box is typically lower than the frequency band for signals from the cable modem. The set top box may require less attenuation than the cable modem, so that applying the required amount of attenuation for return path signals from the cable modem would overattenuate the return path signals from the set top box.
A simple approach known in the prior art to provide attenuation to the return path is the use of a high-pass filter. A high-pass filter can attenuate the lower frequency return path signals but pass higher frequency forward path signals without attenuation. A high-pass filter is disadvantageous because outside of a transition range it provides either very large attenuation or no attenuation at all to a signal at a particular frequency. Better performance will be achieved if attenuation levels can be selected precisely at each tap. A high-pass filter also cannot provide uniform attenuation across a range of frequencies.
A more sophisticated approach, also known in the prior art, is the use of a step attenuator. A conventional step attenuator allows for selective attenuation in the return path via selection of frequency-dependent or frequency-independent components. Conventional step attenuators used in CATV systems achieve flat loss for the full return path. A conventional step attenuator, however, is not suitable for the situation in which two sources of return path signals requiring different levels of attenuation are present. For example, if a set top box transmits between 10-12 MHz and a cable modem transmits between 20 and 40 MHz, a conventional step attenuator cannot apply loss to signals from the cable modem without applying similar loss to signals from the set top box.
Yet another approach for attenuating return path signals is the use of a windowed filter. A windowed filter is a high-pass filter with a second passband. It is known in the art to design the high-pass filter to pass the forward path frequencies without attenuation and to design the passband to pass the return path frequencies without attenuation. By adding attenuation to the passband, for example through the use of a resistive network, the windowed filter can provide selective loss to the return path signals in the pass band. Yet a windowed filter suffers from at least the same difficulties as the full band step attenuator discussed above. The passband cannot apply different levels of uniform attenuation to different sources of return path signals, and the regions of the spectrum outside of the high-pass region and the passband region are of non-uniform attenuation. A prior art windowed filter thus cannot pass signals on the return path from a set top box without attenuation while simultaneously passing signals on the return path from a cable modem with uniform attenuation.